In order to improve the capacity and spectrum usage rate of optical communications, coherent OFDM has attracted the interests of the researchers recently. An OFDM superchannel comprises a plurality of optical carriers, each of the optical carriers individually being modulated, forming several sub-channels, with these sub-channels jointly constituting a superchannel. Each of the optical carriers may use a high-level modulation format (such as QPSK).
In such a system, an interval between subcarriers equals to a baud rate, and severe overlapping exists in the spectrum of each of the sub-channels; however, due to the orthogonality between them, theoretically, each of the sub-channels can still be demodulated and does not interfere with each other. At a receiving side, after being frequency mixed with a local oscillator laser, an optical signal is converted into an electrical signal by an optoelectric detector, and after analog-digital conversion, the electrical signal is subjected to a series of digital signal processing (DSP). DSP comprises subcarrier separation, adaptive equalization, frequency offset compensation, phase recovery etc., and compensates for such effects as transmission damage, frequency offset, line width, etc., and finally recovers the transmitted data. The DSP technique used in a receiver is one of the key techniques of the superchannel. A method reported in an existing document is to use a plurality of local oscillator lasers to target at different subcarriers respectively, so as to receive the subcarriers, and individually perform DSP processing on each of the received subcarriers to demodulate the data.
FIG. 1 is a schematic diagram of the structure of a superchannel receiver of the prior art. As shown in FIG. 1, the receiver divides a signal into multiple paths by using a coupler after receiving the signal from a transmitter. As shown in FIG. 1, a signal branch corresponding to a path of signal may comprise a front end processing device 101 and a signal compensating device 102. Wherein, the front end processing device 101 may comprise a local oscillator (LO) laser 1011, a mixer (HB) 1012, an anti-aliasing filter (AAF) 1013, an analog digital converter (ADC) 1014, a captive dispersion compensator (CDC) 1015 and an adaptive equalizer (AEQ) 1016. And the signal compensating device 102 may comprise a frequency offset compensator (FOC) 1021, a carrier phase recoverer (CPR) 1022 and a data recoverer (DR) 1023.
As shown in FIG. 1, each path of signal may correspond to a local oscillator laser 1011. In a practical system, a plurality of local oscillator lasers may come from an independent laser, and may also come from a multicarrier light source (in non-patent document 1), or a mode-locked laser, etc. In general, it may be a certain laser source containing a plurality of discrete wavelengths.
As shown in FIG. 1, a superchannel optical signal is respectively mixed with local oscillator lasers with different wavelengths, and may be converted into a baseband electrical signal by an optoelectric detector in the mixer 1012. When only one subcarrier is demodulated in each time of sampling, a plurality of local oscillator light sources respectively target to frequencies of a plurality of subcarriers. And then the electrical signals are converted to a digital domain after being sampled by the anti-aliasing filter 1013 and the analog digital converter 1014, and are subsequently subjected to a series of digital signal processing.
Wherein, the captive dispersion compensator 1015 is configured to compensate for the accumulated dispersion after long-haul optical fiber transmission; the adaptive equalizer 1016 is configured to compensate for residual dispersion, polarization mode dispersion and other dynamic linear damages; the frequency offset compensator 1021 is configured to compensate for the frequency offsets of the lasers at the transmitting side and the receiving side; the carrier phase recoverer 1022 is configured to compensate for the phase noise of the lasers at the transmitting side and the receiving side; and the data recoverer 1023 is configured to recover the transmitted data. FIG. 1 shows the case where only one subcarrier is demodulated in each time of sampling, and multiple subcarriers may also be demodulated in each time of sampling.
FIG. 2 is another schematic diagram of the structure of a superchannel receiver of the prior art. As shown in FIG. 2, the front end processing device 201 further comprises a subcarrier separator (SCS) 2010, by which an input signal is separated into two carriers. However, it is not limited thereto, and the input signal may be separated into more carriers, and the numbers of the carriers separated by the subcarrier separators may be identical or different. The number of carriers separated by the subcarrier separator of each branch depend on the system design, and how many branches the whole superchannel is separated into for reception and how many carriers in each branch in which data are recovered should be specified in the system design.
In the implementation of the present invention, the inventors found that the cost of the code error rate can only be lowered to an acceptable level when the sampling rate is very high. As the baud rate of an optical sub-channel is relatively high (at a magnitude of 10 GHz), the sampling rate of an existing analog digital converter cannot satisfy such a requirement. Hence, simple use of methods in which sampling rate is increased is greatly restricted.
In the prior art, digital signal processing on subcarriers is performed independently. Such a method of independently processing subcarriers is at a certain cost of code error rate, and such independent processing cannot eliminate the interference between the subcarriers, and the performance of the system is hence affected.
Non-patent document 1: Benyuan Zhu et al, Ultra-long-haul transmission of 1.2 Tb/s multicarrier no-guard-interval CO-OFDM superchannel using ultra-large-area fiber, Photonics Technology Letters, vol. 22, no. 11, p. 826, 2010;
Non-patent document 2: S. Chandrasekhar et al, Terabit superchannels for high spectral efficiency transmission, ECOC 2010, Tu.3.C.5;
Non-patent document 3: S. Chandrasekhar et al, Experimental investigation on the performance of closely spaced multi-carrier PDM-QPSK with digital coherent detection, Optics Express, vol. 17, no. 24, p. 21350, 2009; and
Non-patent document 4: S. J. Savory et al, Transmission of 42.8 Gb/s polarization multiplexed NRZ-QPSK over 6400 km of standard fiber with no optical dispersion compensation. OTuA1, OFC 2007.